This invention relates to a semiconductor device having a surface passivation layer formed on one major surface of a semiconductor substrate. In the past, surface passivation layers have been generally formed by the vapor growth method or by thermal oxidation.
The most common form of passivation layer in the past has been a silicon dioxide deposited layer on a surface of a semiconductor substrate, the substrate usually being monocrystalline silicon. More recently, it was suggested that the passivation characteristics could be improved by depositing a second layer of silicon nitride on the silicon dioxide layer.
In our copending application, Ser. No. 561,532, it was disclosed that a greatly improved passivation layer could be obtained by depositing a polycrystalline layer directly on the surface of the substrate, which polycrystalline layer contained oxygen in the range between 2 to 45 atomic percent. The silicon dioxide layer would then be deposited on the polycrystalline layer.
In our copending application, Ser. No. 624,889, filed Oct. 22, 1975, we disclosed a semiconductor device in which a first layer of polycrystalline silicon containing oxygen in the range of between 2 to 45 atomic percent, then a layer of polycrystalline containing nitrogen doping of more than 10 atomic percent, and finally covering the nitrogen doped polycrystalline layer with a silicon dioxide layer. It was noted that a nitrogen doped polycrystalline film has much higher resistance to moisture than does either the oxygen doped polycrystalline layer or the silicon dioxide layer.
When only a pure polycrystalline film is used as a passivation layer, its electrical resistance is not high enough to prevent leakage near the surface. In this sense, a polycrystalline layer is not as good as a silicon dioxide layer. All silicon atoms have surface states (sometimes commonly referred to as dangling bonds) at the interface surface, between the substrate and the passivating layer, and this causes some leakage current due to generation-recombination, which leakage current is in addition to the leakage current due to the relatively low resistivity of the polycrystalline layer.
Another type of passivation layer which has been suggested in the past is one in which an amorphous film is used instead of a polycrystalline layer, but this type film has the same objectionable characteristics as a SiO.sub.2 film.